Voice communications over the public switched telephone network (PSTN) have traditionally been limited in bandwidth to the frequency range of 300-3400 kHz. New networks for voice communications, such as cellular telephony and voice over IP (Internet Protocol, VOIP), may not have the same bandwidth limits, and it may be desirable to transmit and receive voice communications that include a wideband frequency range over such networks. For example, it may be desirable to support an audio frequency range that extends down to 50 Hz and/or up to 7 or 8 kHz. It may also be desirable to support other applications, such as high-quality audio or audio/video conferencing, that may have audio speech content in ranges outside the traditional PSTN limits.
Extension of the range supported by a speech coder into higher frequencies may improve intelligibility. For example, the information that differentiates fricatives such as ‘s’ and ‘f’ is largely in the high frequencies. Highband extension may also improve other qualities of speech, such as presence. For example, even a voiced vowel may have spectral energy far above the PSTN limit.
One approach to wideband speech coding involves scaling a narrowband speech coding technique (e.g., one configured to encode the range of 0-4 kHz) to cover the wideband spectrum. For example, a speech signal may be sampled at a higher rate to include components at high frequencies, and a narrowband coding technique may be reconfigured to use more filter coefficients to represent this wideband signal. Narrowband coding techniques such as CELP (codebook excited linear prediction) are computationally intensive, however, and a wideband CELP coder may consume too many processing cycles to be practical for many mobile and other embedded applications. Encoding the entire spectrum of a wideband signal to a desired quality using such a technique may also lead to an unacceptably large increase in bandwidth. Moreover, transcoding of such an encoded signal would be required before even its narrowband portion could be transmitted into and/or decoded by a system that only supports narrowband coding.
It may be desirable to implement wideband speech coding such that at least the narrowband portion of the encoded signal may be sent through a narrowband channel (such as a PSTN channel) without transcoding or other significant modification. Efficiency of the wideband coding extension may also be desirable, for example, to avoid a significant reduction in the number of users that may be serviced in applications such as wireless cellular telephony and broadcasting over wired and wireless channels.
Another approach to wideband speech coding involves coding the narrowband and highband portions of a speech signal as separate subbands. In a system of this type, an increased efficiency may be realized by deriving an excitation for the highband synthesis filter from information already available at the decoder, such as the narrowband excitation signal. Quality may be increased in such a system by including in the encoded signal a series of gain factors that indicate a time-varying relation between a level of the original highband signal and a level of the synthesized highband signal.